An internal-combustion engine of an automobile etc. includes a camshaft rotationally driven in synchronization with rotation of an engine, a cam fixed to the camshaft, and a rocker arm disposed between the cam and a valve stem of an intake valve (or exhaust valve) swingably with a hydraulic lash adjuster used as a fulcrum. In this engine, a drive force from the camshaft is transmitted through the cam and the rocker arm to the valve stem so that the valve stem is displaced, and this displacement of the valve stem causes the intake valve (or the exhaust valve) to open/close. In this operation, the hydraulic lash adjuster automatically adjusts a valve clearance between the cam and the rocker arm in accordance with the drive (stretching and contracting operations) thereof.
In general, as described in Patent Document 1, such a hydraulic lash adjuster has a hollow plunger slidably fitted into a bottomed cylindrical body with a head section thereof projected from an opening side of the body; the plunger has a low pressure chamber formed to store an oil by using an inside thereof and a communication hole formed in the head section thereof to allow communication between an inside and an outside of the low pressure chamber; the body has a high pressure chamber formed therein and filled with an oil between a bottom section of the body and a bottom section of the plunger; respective circumferential wall sections of the body and the plunger are each provided with an oil supply hole for making up an oil supply passage supplying the oil into the low pressure chamber; a biasing means is interposed between the bottom section of the body and the bottom section of the plunger; and a valve mechanism is disposed on the bottom section of the plunger and opens to allow the oil to flow from the low pressure chamber into the high pressure chamber when the plunger performs a stretching motion based on a restoring force of the biasing means.
By using this hydraulic lash adjuster for a valve moving mechanism in an internal-combustion engine, the rocker arm can swingably be supported at the head section of the plunger and, when the plunger receives an input load from the rocker arm with an oil pressure in the high pressure chamber and performs a contracting motion, a damping force can be generated. On the other hand, when a valve clearance is generated, the plunger performs a stretching motion in a direction (a stretching direction) of eliminating the valve clearance.
When the engine is operated, the hydraulic lash adjuster as described above is supplied with an oil pumped out from an oil pan by an oil pump through an oil passage and the low pressure chamber to the high pressure chamber. Therefore, the oil may have air entrained on the way as air bubbles and, if the air is accumulated in the high pressure chamber, the air is compressed in a contracting motion of the plunger and facilitates the contracting motion of the plunger, impairing a basic function of applying a damping force to the plunger.
In view of such situations, a hydraulic lash adjuster is recently proposed so as to prevent air bubbles made of air in the oil from flowing into the high pressure chamber as described in Patent Document 2 and the hydraulic lash adjuster has a cylindrical guide tube additionally disposed in the plunger such that the oil flowing from the oil supply passage into the plunger is guided to the head section of the plunger by using an annular space between an outer circumferential surface of the guide tube and an inner circumferential surface of the plunger. Specifically, a tube with a tip-side outer diameter shorter than a base-side outer diameter is prepared as the guide tube, and a base-side outer circumferential surface thereof is fitted to and retained by a plunger inner circumferential surface on the bottom section side of the plunger relative to the oil supply passage with the tip side of the guide tube directed toward the head section side of the plunger, so as to form the annular space between the inner circumferential surface of the plunger and the outer circumferential surface of the guide tube on the head section side of the plunger relative to the oil supply passage. As a result, when the engine is operated, the oil supplied from the oil supply passage is first guided through the annular space toward the head section side of the plunger, and the air is gradually separated from the oil in the head section of the plunger (coarsening of air bubbles). While the oil itself flows through the inside of the guide tube toward the high pressure chamber as the valve mechanism opens, the separated air is discharged from the communication hole in the plunger head section along with a surplus oil to the outside. Consequently, even if an oil with air entrained as air bubbles is supplied into the plunger, the air in the oil can be restrained from flowing into the high pressure chamber.